Introduction The work outlined in this proposal builds on recent data from our laboratory linking DNA replication and DNA repair to Elys, a protein required for nuclear pore complex assembly in a wide range of organisms. We recently discovered that mutation of Elys reduces levels of an essential component of the Mcm2/7 DNA replication helicase, Mcm2, in rapidly proliferating tissue progenitor cells. We believe that depletion of one of the six Mcm proteins (Mcm2) that comprise the hexameric Mcm2/7 complex accounts for progenitor cell defects in the intestine of zebrafish elys mutants. This discovery represents a potentially unique link between the nuclear pore complex and DNA replication in vertebrate stem/progenitor cells and as such, supports a fundamental premise of our laboratory[unreadable]s research effort;that unbiased genetic screens in the zebrafish can promote greater understanding of essential biological processes that have relevance for human biology and disease. Our preliminary analyses of the zebrafish elys mutant lead to the hypothesis that Elys regulates Mcm2- chromatin interactions, either on its own or within the nuclear pore complex, and thereby plays a role in the response of proliferating cells to stress incurred during DNA replication;i.e. [unreadable] replication stress. Because unresolved replication stress can lead to mutations that culminate in progenitor cell senescence and or cancerous transformation, the studies outlined in this proposal are likely to have direct clinical relevance. This proposal consists of three interrelated aims. The goals of the first aim are to determine whether Elys[unreadable]s interaction with Mcm2 can be dissociated from its established role in nuclear pore complex assembly, and to determine how Mcm2 deficiency contributes to the Elys tissue progenitor cell phenotype. The goal of the second aim is to conduct in vivo structure-function analyses in the zebrafish to further define the biochemical basis of the physical interaction between Elys and Mcm2. The goal of the third aim is to explore the role of Elys-Mcm2 interactions in the mammalian intestine by studying the phenotype arising from conditional Elys disruption in intestinal epithelia of genetically modified mice, and the effect of Elys deficiency on the intestinal phenotype of hypomorphic Mcm2 mouse mutants that develop intestinal polyps and progenitor cell defects.